Various devices are known for determining the water weight in the forming section. However, these device are limited in that the water weight is indirectly determined, i.e., calculated from the measurement of another parameter. One such device (Honeywell SpectraFoil) calculates the water weight based upon a measurement of the conductance of the stock layer on the former. This device measures the electric current through the stock and is sensitive to chemical changes in the stock.
Another method described in U.S. Pat. Nos. 3,442,756 and 4,789,431 provides an ultrasonic measurement of the stock layer thickness by measuring reflectance of the ultrasonic sound from the top and bottom surface of the stock. Thus, this device provides essentially a “layer-thickness-measurement,” however, it cannot be used after the dry line.
Another device provides a gamma backscatter based total mass measurement. However, this device is disadvantageous in that measurement noise is high and that the measurement requires the use of radioactive material.
In contrast to the foregoing devices, a single sided resonant microwave cavity sensor (ScanPro by Lorentzen & Wettre) is known, which more or less directly measures water weight. However, the size of these devices do not match the space requirements for paper making installations.
A further measurement method, which uses microwaves for measuring water weight in the forming section of the paper machine, has been described in detail in European Patent Application No. EP 0 903 657. While the described method uses a sensor that measures the moisture content (water weight) through the sheet supporting fabric, none of the commercially available microwave sensors are suited for this application, i.e., as noted above, the sensors such as the ScanPro do not match the space requirements.
Other commercially available microwave sensors, which are even smaller than the above-discussed ScanPro, are only able to sense a very thin layer of water above the sensor surface. Thus, these sensors are not suited for measuring water through the forming fabric.
Another microwave moisture content and weight of sheet device is known from U.S. Pat. No. 4,297,874, which utilizes a resonant cavity. In this device, the sheet passes through the resonant cavity to monitor attenuation or phase shift. However, like the other noted devices, the resonant cavity is too large for satisfactory use in the forming section.